Technical Field
Aspects of the embodiments relate generally to sinusoidally driven motors, and more specifically to systems, methods, and modes for controlling sinusoidally driven motors to achieve efficient motion and reduced noise.
Background Art
Motorized window treatments provide a convenient one-touch control solution for screening windows, doors, or the like, to achieve privacy and thermal effects. Various types of motorized window treatments exist, including motorized roller shades, inverted rollers, Roman shades, Austrian shades, pleated shades, blinds, shutters, skylight shades, garage doors, or the like. A typical motorized window treatment includes a shade material that is manipulated by the motor to cover or uncover the window.
Such motorized applications require high performance motors capable of being driven with the least amount of audible noise possible, while maintaining stable velocity, position control, and energy efficiency. Generally, two types of motor controls are utilized, including linear and nonlinear.
In linear control, the controller directly controls the motor via a control signal. Linear control methods may employ some type of linear mathematical compensator that is fine-tuned with the internal parameters of the system being controlled. A linear system operates without any awareness of extraneous factors. When these methods are implemented using the digital domain, they will have certain impulse reactions when unexpected physical transients occur, such as friction, component wear, changes in temperature, and changes in load. While these types of controllers offer system reliability, including in torque output and efficiency, they create speed oscillations that cause audible noise. These speed oscillations can also be observed visually, when for example amplified by a bouncing shade.
Nonlinear control systems modify the output by changes in the input using feedback. These types of systems undertake some dynamic subtleties under certain operating regions, but can potentially get complex to implement and difficult to guarantee total convergence under all the variable operating space. In most cases these systems have no better yield in audible noise than the linear control systems.
For example, referring to FIG. 1, there is shown a simplified depiction of a brushless direct current (BLDC) motor 101. A BLDC motor 101 comprises a rotor 102 having a driving shaft 103 and a permanent magnet 104 divided into one to eight north (N)-south (S) pole pairs. A stator 107 is position about the rotor 102 that generally comprises a plurality of steel laminations that carry phase windings 105a-c defining the stator pole pairs. The BLDC motor 101 operates via electrical commutation generated by a controller 110. Commutation is the process of switching current in the phases in order to generate motion. Current is run through the phase windings 105a-c in alternating directions in a sequence such that the permanent magnet poles follow the revolving magnetic field that is caused by the windings.
To determine the timing of the current running through the phase windings 105a-c, Hall Effect sensors 106a-c are generally placed around the rotor 102 for each phase control to track the position of the rotor 102 and provide feedback to the controller 110. Speed of the rotor 102 is determined by the time interval between signals from the Hall Effect sensors 106a-c. One control scheme for electronic commutation involves sinusoidal commutation. Typically, the controller 110 outputs three sinusoidal waveforms at 120 degrees out of phase across the three phases of the motor 101, as shown in FIG. 2. The phase angle of these sinusoidal waveforms depends on the position of the rotor 102 as reported by the Hall Effect sensors feedback. To maintain constant output speed, as more load is exerted on the motor 101, the controller 110 may change the frequency of the sinusoidal waveform, and thereby change the speed of the motor 101, based on speed errors reported by the Hall Effect sensors 106a-c. Problems can occur if the Hall Effect sensor placement is not accurate with respect to the rotor 102 causing a constant lag and shift in the sinusoidal waveform. This results in falsely detected instantaneous speed changes, timing errors, and torque ripple.
When analyzing the components of an audibly perceived system, the most common trait is the combination of different frequency components with a spread of intensities. For motorized devices and applications, the focus is mainly on frequencies under 1 kHz where most perturbing noises are found. While investigating and researching the source of these frequencies, multiple sources were found that are generic enough to affect many motorized systems. The main sources being the commutation frequency, timing corrections, and the sporadic rate of speed compensation that is generated by the linear and nonlinear behavior of the control algorithms driving the motor control.
Accordingly, a need has arisen for systems, methods, and modes for controlling sinusoidally driven motors to achieve efficient motion and reduced noise without effecting the frequency or speed of the motor.